Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2008
Publication Date: 12/1/2008
Publication URL: http://hdl.handle.net/10113/26988
Citation: Allred, B.J. 2008. Cation Effects on Nitrate Mobility in an Unsaturated Soil. Transactions of the ASABE. 51(6):1997-2012. Interpretive Summary: Nitrate can cause degradation in the quality of ground and surface waters. After fertilizer is applied at the ground surface, initial nitrate movement is through the soil profile before percolating downwards to underlying aquifers or being captured by drainage pipes and discharged into local waterways. Consequently, a better understanding is needed of the processes affecting nitrate movement in unsaturated soil. Laboratory unsaturated soil column tests were conducted to determine whether the type of cation (positively charged ion) present with nitrate has an impact on soil nitrate mobility. Test results showed that nitrate is most mobile when the cation present has a positive charge of one, less mobile when the cation present had a positive charge of two, and least mobile when the cation present has a positive charge of three. These experimental results provide valuable information for better management of farm field fertilizer applications and for improving computer modeling software employed to simulate nitrate transport through the soil profile.
Technical Abstract: Transient, unsaturated, horizontal column experiments were carried out with a computer controlled syringe pump for the purpose of assessing counterion (accompanying cation) effects on nitrate (NO3-) mobility in unsaturated soil. A loam soil with calcium dominated cation exchange sites was used in all column experiments. Duplicate tests were conducted with six different injection solutions applied at the inlets of relatively dry soil columns (initial volumetric water content averaged 0.012). All six injection solutions contained 0.0142 moles/L NO3- (200 mg/L NO3--N), but differed from one another with respect to the counterion present, either K+, NH4+, Ca2+, Mg2+, Al3+, or Fe3+. Boundary conditions at the column inlet remained constant throughout the time duration of the test, and additionally, the soil adjacent to the inlet had the greatest amount of injection solution flushing; therefore, column inlet volumetric water content and soil solution NO3--N concentration measurements were employed to quantify counterion impacts on soil nitrate mobility under unsaturated flow conditions. The inlet volumetric water content was fairly constant for all column tests (average = 0.375, standard deviation = 0.005). Average soil solution NO3--N concentrations at the inlet were 165.3, 167.8, 175.7, 174.4, 190.2, and 214.2 mg/L, respectively, for tests conducted with injection solutions containing K+, NH4+, Ca2+, Mg2+, Al3+, and Fe3+. Consequently, based on a NO3--N injection concentration of 200 mg/L, the tests using injection solutions containing monovalent cations (K+ and NH4+) had the largest inlet NO3- anion exclusion, while inlet NO3- anion exclusion was significantly less for tests using injection solution containing divalent cations (Ca2+ and Mg2+). Inlet NO3- anion exclusion was minimal for the tests with injections solutions containing the trivalent cation, Al3+, and for the tests carried out with injection solutions containing the trivalent cation, Fe3+, inlet NO3- anion adsorption actually occurred. Decreased inlet NO3- anion exclusion for the divalent cation tests compared to the monovalent cation tests was likely due in large part to the divalent cation injection solutions having a greater ionic strength than the monovalent cation injection solutions. Soil pH reductions at the column inlet probably account for the minimal NO3- anion exclusion or NO3- anion adsorption found with tests conducted using trivalent cation injection solutions. These experimental results provide valuable information for better management of farm field fertilizer applications and for improving computer modeling software employed to simulate nitrate transport through the soil profile.